Boiler control apparatus



Dec. 2, 1969 c. E. INES 3,481,538

BOILER CONTROL APPARATUS Filed May 2, 1968 l 2 Sheets-Sheet 1 INVENTQR CHARLES: E. MNA-s Dec. 2, C, E, D|NES BOILER CONTROL APPARATUS 2 Sheets-Sheet 2 Filed May' 2, 1968 Il l- L M Humm |.v||| T QN wva 23 AL N: u Aw r|| IG r-- MQ Nw NQ QQ TNS @Q Q1 Wm vm@ n@ WQ H .QN QQ n@ ma? EQ E T INVENTOR WPI-ES E20/NES BY United States Patent O 3,481,538 BOILER CONTROL APPARATUS Charles E. Dines, Lewisham, London, England, assignor to Elliott Brothers (London) Limited, London, England, a British company Filed May 2, 1968, Ser. No. 726,051 Int. Cl. F22b 35/00; F23k 3/02 U.S. Cl. 236-245 15 Claims ABSTRACT OF THE DISCLOSURE Control apparatus for a boiler has rst control means for controlling the air flow to the combustion chamber of the boiler during start-up and low level boiler operation by moving air constrictor means in dependence on the ratio of a fuel demand signal, and air flow rate, and second control means for controlling the air flow during on-load operation by moving the air flow constrictor means in dependence on the rate of steam output owrate of the boiler and air ow rate, the second control means being inoperative to control the ow constrictor in dependence on said steam/air rates until a predetermined level of boiler operation is reached.

Summary of invention The control apparatus of the invention is for use in a boiler plant, in particular using pulverised fuel, in which combustion air is supplied to the combustion chamber of the boiler through an air duct containing a forced draught fan and flow constrictor means in the form of movable inlet guide vanes of the fan, and a movable damper downstream of the guide vanes. Under start-up and low level boiler operation, the guide vanes are largely closed so that the damper operates on a relative low air flow.

The control apparatus includes a tfuel demand signal generator, in particular controlled by the pressure in a steam output pipe, and a fuel control device responsive to the fuel demand signal. The constrictor means is controlled to maintain a balance between combustion air flow rate and a fuel demand signal.

A second air control assembly is operable to move the flow constrictor means to maintain a balance between combustion air flow rate and steam ow rate. A selector which is responsive to the transition from low-level to onload boiler operation, operates to render the flow constrictor means responsive to the steam/ air rates va the second air control assembly.

The combustion air duct can be branched downstream of the fan to form a number of windboxes each having its own damper and fuel supply.

In particular the fuel supply or each fuel supply is a fuel-pulverising mill supplied with an air flow for carrying pulverised fuel into the combustion chamber.

Description of drawing Two forms of control apparatus according to the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which:

FIGURE 1 shows combustion start-up and on-load control apparatus for a pulverised fuel boiler plant having a -windbox common to all burners of the plant and FIGURE 2 shows combustion start-up and on-load control apparatus for a pulverised fuel boiler plant having a number of mills each associated with a group of burners and a separate windbox.

Patented Dec. 2, 1'969 Description of preferred embodiment It will be appreciated that the drawings show, and following descriptions refer to, simplified plant and that in actual plant the auxiliaries may for example include at least two forced draught fans, two or more primary air systems and six or eight mills. It will also be appreciated that the control apparatus for the actual plant would be similar to that described with reference to the simplied plant.

The construction of boiler plant incorporating primary and secondary air flow systems in which the primary air flow controls the fuel output from mills to combustion chambers is well known to those skilled in the art and is therefore not described in detail in this specification.

Individual components, for example, the various regulators, repeaters and summator, of the preferred embodiment will not be described in detail in cases where they are of types well known to those skilled in the art or are of such a nature that their construction is readily apparent to those skilled in the art.

In FIGURE 1 a masterv regulator 1 is arranged to measure the steam pressure at a point 2 in the steam main 3 from a boiler 50. A desired steam pressure setting of the regulator 1 is adjusted by a servomotor 4 either manually from a set point station 5 or automatically according to a preset pressure raising routine from a timed programmer 46. Deviation of the measured steam, pressure from the desired steam pressure causes the regulator 1 to produce an electrical output signal 7 to take corrective action upon a fuel throughput regulator 8 for a fuel mill 9 which via fuel pipes 10y delivers pulverised fuel to burners 11 in a combustion chamber 12.

The output signal 7, which represents total fuel demand, is applied to a signal repeater 13, including a regulator which is connected by a separate output line to each mill. The repeater, in response to the signal 7, sends a separate equal electrical signal 16 to each mill 9 (one only being shown) which is in operation. By means of an electrical feedback signal 14, from a summator 15 connected to each repeater line and thus responsive to all the signals 16, the repeater 13 maintains the sum effect of the signals 16 in balance with the output signal 7. The feedback signal 14 represents the total value of those signals 16 which are communicated to the summator 15 by electrical closure of electrical switches connecting the repeater output lines to the summator 17. The switches 17 are relay operated by auxiliary contacts in switchgear (not shown) that controls the running of the mills, each switch 17 being associated with a different mill 9, and controlled by adjustable time delay relays so that they are closed only when the respective mill is capable of delivering pulverised fuel substantially in accordance with primary air flow demand. The fuel throughput metering system is thus not falsified by mill and fuel pipe purging routines. The feedback signal 14 ensures that the individual signals 16 are each equivalent in terms of fuel throughput to the output signal 7 divided by the number of mills 9 in operation and that the signals 16 become readjusted automatically as mills 9 are brought into or taken out of service.

The regulator 8 adjusts a primary air ow control damper 18 to produce a balance between the differential pressure of primary air W measured by a venturi measuring device 19 and the associated signal 16.

In a start-up mode of operation of the plant, the ratio of combustion air ow to fuel flow is controlled by an electrical 'signal 20 generated by the signal repeater 13 in parallel with the signals 16 and having a Iiixed relationship to fuel throughput per mill 9 and therefore to the magnitude of the signals 16.

A panel-mounted station 21 provides bias of this signal 20 for adjusting the fuel/air ratio to suit variations occurring in the quality of the plant fuel supply. The resulting signal 22 forms a set point for a first air control means in the form of a windbox pressure regulator 23 which generates a control signal 24 to adjust, by way o'f a servomotor 25, a low load secondary-air-ow control damper 26, located in series with a forced draught fan 27, so that pressure in the burner Windbox 28, as sensed and transduced by the measuring diaphragm 29, is maintained in balance with the signal 22.

In order to facilitate adjustment of the combustion air to low flow rates (compared to on-load flow rates) by the damper 26, the fan inlet guide vanes 30, which are used to control total air quantities in the normal on-load range, must be positioned for minimal air delivery. The vanes 30 are closed and held closed by a servomotor 31 under the control of the regulator 23 by way of electrical signals 32 and 33 until the throughput rate demanded by the signal 22 increases to result in the damper 26 opening to the limit for effective control. At this point the regulator 23 is arranged to take action on signal 32 so that the demanded windbox pressure is achieved substantially by adjustment of the vanes 30.

As the boiler throughput is increased and enters the normal operating range, in which combustion can be controlled by means of total combustion air and steam flow measuring loops employing inferential measuring methods, a second air control means in the form of a steam flow air flow ratio regulator 34 is released from a null state by a selector means, in the form of a solenoid device 35, operated from a switching relay 36, which is unlatched by a limit switch 37, at a given position of the vanes 30. Control of the vanes 30 is returned to the regulator 23, only when the boiler load is reduced to a level at which a further limit switch 38 is closed and is held closed for an interval, as set by a time delay relay 39, long enough to cause the relay 36 to become energized thereby to operate the solenoid device 35 to return the ratio regulator 34 to the null state.

When released, the ratio regulator 34 is arranged to add to or substract from the signal 32, produced by the regulator 23, as required to control the air flow rate by means of the vanes 30 to achieve a balance of the ratio regulators measuring element 40. The element 40 is arranged to compare total combustion air flow with total steam flow measurement. The steam flow is inferred from a differential pressure produced by a restriction 41, in the steam main 3, in conjunction with a transmitting device 42 which gives an electrical output signal 43 lrepresenting steam flow rate. Bias of the output signal 43 is afforded by a manually operated control station 44 thereby to vary the ratio between steam ow and total combustion air flow to obtain a desired excess air margin to operating requirements. The output signal 45 from the control panel station 44 adjusts the set point of the measuring element 40 of the regulator 34 against which acts differential pressure, representing total combustion air flow rate, produced by a venturi contraction 47 formed in the forced draught fan ducting on the suction side of the fan 27 and transduced by a measuring diaphragm 46.

In the embodiment of FIGURE 2, a master regulator 101 measures the working pressure at point 102 in a steam main 103 from a steam generator 150. Deviation from the desired value of steam pressure, as adjusted yby a servomotor 104 from a manually operated set point station S, produces an electrical output signal 106, to take corrective action upon fuel throughput regulator 107, thereby to appropriately adjust throughput of a mill 108 which, via fuel pipes 109, delivers pulverised fuel to burners 110 in combustion chamber 111. Fuel pipes 109g and burners 110:1 are associated with another mill (not shown) which is also controlled by a fuel throughput regulator (not shown) in response to the output signal 106.

The signal 106, representing total fuel demand is applied to a signal repeater 112, which generates a separate electrical signal 113 for each mill (one only being shown) and which by means of a feed back signal 114, from a summator 115, representing the total value of as many of the several signals 113 as are lmade active by closure of electrical switches 116, causes the signal repeater 112 to maintain the combined throughput of the active mills in balance with the demand signal 106.

The switches 116 are relay operated from auxiliary contacts on the switchgear that controls the running of the mills raw coal feeders with suitable time delay so that they are closed only when the respective mill is capable of delivering pulverised fuel substantially in accordance with the fuel demand signal (signal 113).

Each fuel regulator 107 adjusts the primary air flow control damper 117 so that a differential head is produced by a venturi metering device 118 of a magnitude to achieve a balance with the associated demand signal 113. The demand signal is transduced -by servomotor 119 into a force on a weighbeam 120 which forms part of each said fuel regulator 107.

In the normal on-load operating range of the boiler, combustion is optimised by means of control by a second air control means in the form of a steam/ air ratio regulator 121 which generates an electrical output signal 122 t0 control, via a transmission and distributional system, dampers 123 which regulate a supply of secondary air in ducts 124, to windboxes 125 (two only being shown) to achieve balance of its measuring element 126, which compares total air flow rate with total steam flow rate. Steam dow is inferred from the differential head produced by a restriction 127 in steam main 103, in conjunction with a transmitting device 128 which gives an output signal 129 in known relationship to flow rate. Proportional bias of this signal is afforded by a manually operated station 130, so that the ratio to be observed between steam/ air flow can be varied to obtain a desired excess air margin. The output signal 131 of the station 130 forms the set point of the regulator 121, and is balanced by the differential head produced by a venturi contraction 133, formed in inlet ducting 134 to a forced draught fan 135, by way of differential pressure transducer 132.

The power control means of the forced draught fan 135, in the form of inlet guide vanes 136, is modulated to maintain sufficiency of delivery pressure for a given mill throughput by a demand signal 137, which is generated by the signal repeater 112, in parallel with and as the square of the fuel signals 113, to form a set point of a first control means in the form of a forced draught pressure regulator 138. This regulator generates a signal 139 to control, by means of a servomotor 140, the position of the inlet guide vanes 136, to maintain pressure at a point 141, in the hot air main 142, in balance with the demand signal 137.

The relationship between the demand signal 137, and the measured pressure value at point 141, is modified by reset action initiated from limit switches 143 set on the operating motion of each of the dampers 123, to observe the desired maximum and minimum states and a given margin of damper travel to afford switching stability.

The reset action is typically applied by ratio adjustment within measuring element 144, of regulator 138, as affected by a reversing motor 145, which is made to run in discrete steps by a rate adjusting means in the form of a pulse generator 146, that is adjustable in frequency and duration of both made and break periods so that the reset rate can be optimised to the time constants of the forced draught plant.

The groups of limit switches 143, are integrated in a logic relay system (not shown).

The transmission and distributional system for secondary air, mentioned previously, will now be described.

The output 122, generated by the steam/ air ratio regulator 121, is in the nature of an error signal in that it functions to take action on the secondary air supply to prevent departure from a desired steam/air ratio. This action is shared equally among the several secondary air control loops by a signal repeater 147, which produces the required number of separate electrical signals 148.

Secondary air to each windbox is controlled by an air ow regulator in the form of a secondary air regulator 151 in approximate ratio with the respective fuel throughput in order to cater, for example, when the number of mills in service is changed. The signal repeater 147 and air regulators 151 together form a distribution means.

To achieve this control, proportional bias is applied to signal 148, to produce a desired value electrical signal 150, by a potentiometric divider 149, the slider of which is driven from the servomotor 119, that transduces the fuel demand signal 113 into a loading force on the fuel regulator 107.

Each air regulator 151 controls a damper 123, in an individual duct 124, to an independent windbox 125, by means of a servomotor 152, to maintain the ow rate, as measured by a pressure difference device 153, in balance with the respective value signal 150.

It will be seen that, by imposing the steam/air control mode in this manner on a discrete number of tiring units, each having basic fuel and air control loops as described, a fuel/ air ratio control mode for start-up and low-load operation is readily achieved.

When operating below the normal on-load range where combustion can be optimised by the steam-air mode, the regulator 121 is disconnected from its measuring element 126 and made to assume a predetermined position. This is effected by a solenoid device 154 which is operated manually from a panel switch 155, or automatically at a preselected point in the load range by a switching relay switch 156, the switch being operated from the steam flow transmitter output signal 129.

The steam/ air ratio regulator 121, when in the fixed position, generates signal 122, at a constant value which is chosen according to the increase in excess air margin considered necessary to safeguard the uncertainty that exists in the fuel metering system, with respect to caloric value, in the absence of the steam/ air mode.

The boiler operator is provided at this time with the means of setting the approximate fuel/ air ratio by a panel station 157, which effects reduction of the standing signal 122, within a prescribed safe limit. This means is switched into and out of effect simultaneously with the operation of the solenoid device 154, by the parallel circuit 158, in order to preserve the ratio setting established in one period of start-up mode operating to the next i.e. excluding the possibility of malfunctioning of the station 157.

The resulting output signal is re-transmitted by the signal repeater 147 to form the individual basic demand signals 148, that are modified to suit the individual fuel throughput level by the ratio bias 149, to produce the respective demand signal 150, the signal 150 is transduced into a loading force on the appropriate air regulator 151, by a servomotor 159, that would perform the squaring function necessary in the example shown where the measured value represents the secondary air flow velocity head.

At start-up of the boiler, the pressure at point 102 in the steam main 103 will be below the set point value applied to the master regulator 101, depending on the time elapsing since the boiler was shut down. Consequently, the fuel demand signal 106 will be at the maximum value and hence, if applied to the fuel regulators at this time, would result in full loading of the mills as commissioned.

A maximum signal limiting relay 160 is employed which receives an alternative demand signal 161, and transmits this value to the fuel regulator 107 when it is the lower of the two signals.

The alternative demand signal 161 is generated by .a ramp loading device 162. This device functions to generate an increasing signal at an infinitely variable rate yas selected by the operator Iat station 163. This station displays the output signal 161, has setting means calibrated in terms of percentage heat release and means to start and stop the ramping action. The operator is thus able to demand any firing rate that is below the requirements of the steam pressure element and greater than the turndown limit of the milling plant.

An essential requirement for correct function of the steam/air ratio combustion mode is for the heat release rate to be controlled to maintain substantially constant steam pressure. The availability of the steam/air ratio mode is therefore made conditional upon the existence of the state in which the relay is set to transmit signal 106 to the fuel regulator 107 by an electrical interlock connection 164 with the circuit that controls the operation of the solenoid 154.

With regard to minimal firing rate, the employment of a mill below the given safe turn-down limit is prevented by a minimum loading limiter 165 in the indivadual signal lines 113 to each fuel regulator 107. A similar limiter 166, in signal line 137, is set to prevent the set point on the forced draught regulator 138 from falling below the value necessary to provide suiciency of combustion air supply when the mills are operating at minimal throughput, i.e. in the case when the total fuel demand applying from signal 106, or 161, becomes less than the given control range of the mills in service.

In order to ensure that the set point on the minimum loading limiter 166 is not falsified by start-up or abnormal operating conditions, readjustment of the ratio between set point signal 137 and the measured value on regulator 138 is inhibited in circumstances which do not permit the secondary air dampers 123 to operate at optimum efficiency.

The resetting arrangement is retained in an existing state by cutting the power supply line 167 to the pulse generator 146, by using a switching relay 168. The switching relay 168 is energised by a limit switch 169, set to close at minimal opening of the fan guide vanes 136. When energised, the switching relay 168 latches-in and opens its contacts to interrupt the power supply. At the wider opening of the guide vanes which establishes adequate control of fan delivery pressure, a limit switch 170 operates to unlatch the relay 168, which then restores supply to the pulse generator 146.

The relay 168 will function similarly from signals derived from other states which may not permit the secondary air dampers to operate efficiently as for instance when the forced draught plant is under manual control.

It will be appreciated that in a case Where the function of regulator 138 is performed by a controller of a type which does not afford the facility for adjusting the relationship between set point and measured value internally, the same effect is to be achieved by a signal modulating device employing similar resetting technique to that described. In such -a case a transmitting device would apply the measured value vat point 141 to the said controller and, if this device is of the root extracting type, the demand signal 197 would be generated by `the signal repeater 112, in linear relationship to the fuel demand signals 113.

It will also be appreciated that, in an alternative construction, if the fuel regulator is of a type which does not afford the facility of a servomotor 119 to drive the slider, the slider could be driven by an instrument that measures the differential head produced by the primary air ow device 118. Differences will thus appear in the desired value signal 150, which are applied to the several air flow regulators 151, depending upon differences in the primary air (fuel) distribution pattern.

When operating below the normal on-load range, in an alternative arrangement, the change-over point ymay be made dependent upon the number of mills operating since the number of idle burners is a limiting factor in the use of the steam/.air mode when a large percentage of the 7 combustion air supply does not enter into the combustion process.

I claim:

1. In control apparatus for a boiler plant, the combination comprising first air control means, operative during start-up and low load level of boiler operation, to control combustion air flow rate in response to the ratio of fuel flow rate and combustion air flow rate,

second air control means, operative during normal onload level of boiler operation, to control combustion air flow rate in response to the ratio of steam output ow rate and combustion air flow rate,

sensing means for sensing the load level of operation of the boiler, and

selector means responsive to said sensing means and operable selectively in dependence on the load level of boiler operation to render either the first air control means or the second air control means operative to control the combustion air flow.

2. Control apparatus, for a boiler plant having a combustion air flow duct, a fuel supply and a steam output pipe, the apparatus comprising in combination,

fuel flow control means for controlling the rate of fuel fiow to the plant,

first air diow control means for controlling the rate of combustion air ow through the duct during start-up and low-level boiler operation,

second air flow control means for controlling the rate of combustion air flow through the duct during onload boiler operation,

a fuel ldemand signal generator for generating and supplying a first control signal to said fuel flow control means, and a second control signal to said first air ow control means,

movable air flow constrictor means in the duct,

the first air flow control means comprising first means responsive to air flow rate in said duct to generate a first air flow rate signal, and

a first regulator responsive to the ratio of said air flow rate signal and said second control signal to move said constrictor means in a sense to balance the air flow rate signal against the second control signal,

sensing means for sensing the load level of boiler operation,

the second air flow control means comprising second means responsive to air flow rate in said duct to generate a second air iiow rate signal,

means responsive to steam fiow rate in said steam pipe to generate a steam fiow rate signal,

a second regulator responsive to the ratio of said steam flow rate signal and said second air flow rate signal to move said air ow constrictor means, and

a selector responsive to said load level sensing means to render the second regulator operative to control the air flow constrictor means upon transition from low-level to on-load boiler operation.

3. A method of operating a boiler plant having a fuel supply, a combustion air duct and a steam output pipe which comprises the steps of generating a fuel demand signal,

controlling the supply of fuel to the boiler plant in dependence on said signal,

measuring the combustion air flow in the air duct and controlling said air flow in dependence on the ratio of the measured air flow and the fuel demand signal, during start-up and low level boiler operation,

generating a control signal indicative of the level of boiler operation,

measuring the steam pressure in the steam pipe and, in response to a said control signal indicative of the transition from low level boiler operation to on-load boiler operation, varying the air flow in the duct to maintain a predetermined ratio between the said steam pressure and air flow.

4. A boiler plant comprising in combination a combustion chamber,

a boiler heated by the combustion in said chamber,

a steam pipe connected to said boiler,

a fuel supply connected to said combustion chamber,

a combustion air duct connected to supply air to said combustion chamber,

means for generating a fuel demand signal,

fuel supply control means responsive to said fuel demand signal to control the supply of fuel to the combustion chamber,

a first movable air ow constrictor at a first position in said air duct,

first air control means for controlling start-up and low level boiler operation and comprising means measuring air pressure in sai-d duct downstream of said first position and generating an air flow rate signal, and

a first regulator which responds to the ratio of said fuel demand signal and said air flow rate signal, varies the position of said first constrictor, and generates a control signal indicative of a demand for air iiow in excess of that being supplied to the rst constrictor,

a second movable air ow constrictor in said air duct at a second position upstream of said first position and movable in response to said control signal,

second air control means for controlling on-load boiler operation and comprising means measuring air ow rate entering said second constrictor,

means measuring steam flow rate in said second constrictor,

means measuring steam flow rate in said steam pipe,

a second regulator connected to said air flow rateand steam fiow rate-measuring means and said second ow constrictor and which, when operative, responds to the ratio of steam flow rate and air flow rate to vary the position of said second constrictor, and

a selector connected to the second regulator and responsive to the position of the second movable air flow constrictor, at the transition from low-level to on-load boiler operation, to render said second regulator operative to control said second air ow constrictor.

5. A boiler plant according to claim 4 comprising a forced draught fan in said air duct,

said second air flow constrictor comprising movable fan-inlet guide vanes, and wherein said first air ow constrictor is a movable damper.

6. A boiler plant according to claim 4 wherein the fuel supply comprises a supply of pulverised fuel and a supply of air for carrying the pulverised fuel into the combustion chamber.

7. A boiler plant according to claim 6 wherein the fuel supply control means is operative to control the fuelcarrying air supply.

8. A boiler plant according to claim 4 wherein said combustion air duct is divided downstream of said second constrictor into a plurality of Windboxes, said fuel supply comprising a plurality of fuel supply units each associated with a separate one of said Windboxes, and a plurality of said lirst air control means each associated with a separate one of said windboxes.

9. A boiler plant according to claim 4, wherein the means for generating a fuel demand signal is responsive to steam pressure in said steam pipe.

10. A boiler plant comprising in combination a combustion chamber,

a boiler heated by said combustion chamber,

a steam pipe connected to said boiler,

a fuel supply connected to said combustion chamber, a combustion air duct connected to supply air to said combustion chamber, means for generating a fuel demand signal, fuel supply control means responsive to said fuel demand signal to control the supply of fuel to the combustion chamber, a first movable air flow constrictor (123) at a first position in said air duct, a second movable air ow constrictor (136) in said duct upstream of said iirst constrictor, first air control means for controlling start-up and low level boiler operation and comprising means measuring air pressure in said duct upstream of said first position and generating an air flow rate signal, and a rst regulator (138) which responds to the ratio of said fuel demand signal and said air flow rate signal and to the position of said first constrictor to vary the position of said second constrictor, a second air control means for controlling on-load boiler operation and comprising means (132) measuring air flow rate entering said second constrictor, means (128) measuring steam ow rate in said steam pipe, and a second regulator connected to said air ow rateand steam ow rate-measuring means and which, in a first operative state, emits a constant control signal and, in a second operative state, emits a variable control signal dependent on the ratio of steam flow rate and air flow rate, means (151) connected to the second regulator and responsive to the rate of the air ilow rate through the rst constrictor and said constant or variable control signal to move the first constrictor, and a selector (154) connected to the Second regulator (121) and effective in response to a predetermined fuel demand signal (164) indicative of the transition from low level to on-load boiler operation to render said second regulator (121) operative to emit said variable control signal thereby to control said rst air ilow constrictor.

11. A boiler plant according to claim 10 comprising a forced draught fan in said air duct,

said second air flow constrictor comprising movable fan-inlet guide vanes, and wherein said rst air ow constrictor is a movable damper.

12. A boiler plant according to claim 10 wherein the fuel supply comprises a supply of pulverised fuel and a supply of air for carrying the pulverised fuel into the combustion chamber.

13. A boiler plant according to claim 12 wherein the fuel supply control means is operative to control the fuel-carrying air supply.

14. A boiler plant according to claim 10 wherein said combustion air duct is divided downstream of said second constrictor into a plurality of windboxes, said fuel supply comprising a plurality of fuel supply units each associated with a separate one of said windboxes, and a plurality of said first constrictor control means (151) each associated with a separate one of said windboxes.

15. A boiler plant according to claim 10 wherein the means for generating a fuel demand signal is responsive to steam pressure in said steam pipe.

References Cited UNITED STATES PATENTS 2,292,023 8/ 1942 Dickey 1,22--504 2,623,698 12/1952 Dickey 236-14 EDWARD I. MICHAEL, Primary Examiner U.S. Cl. X.R. -103 

